Process for the manufacture of an aldehyde

ABSTRACT

Manufacture of p-tert. butyl-benzaldehyde (TBT) by oxidation with a Mn 3+   salt, which salt is generated by electrochemical oxidation of a Mn 2+   salt, and whereby the chemical oxidation and the electrochemical oxidation are carried out in separate reaction vessels.

The invention is concerned with a process for the manufacture ofp-tert.butylbenzaldehyde (TBB). This aldehyde is a known substance,especially as an intermediate.

The process comprises oxidizing p-tert.butyltoluene (TBT) with a Mn³⁺salt, which salt is generated by electrochemical oxidation of a Mn²⁺salt, and whereby the chemical oxidation and the electrochemicaloxidation are carried out in separate reaction vessels.

The sulphate is preferably used as the manganese salt. However, thephosphate can also be used. This salt gives no problems at all withrespect to electrochemical stability (i.e. cathodic as well as anodic),with respect to interference with organic materials and with respect tocorrosion.

The electrochemical oxidation is conveniently carried out in moderatelyconcentrated sulphuric acid, namely in 40% (5.3 molar) to 90% (16.6molar), especially in 50% (7.1 molar) to 65% (10.3 molar), sulphuricacid.

The electrochemical oxidation is conveniently carried out at an elevatedtemperature, preferably in a temperature range of about 60° C. to 110°C., especially at 80° C. to 100° C. and particularly at about 85° C.

The concentration of the manganese salt in the inorganic phaseconveniently amounts of 1 to 5 mol/l, especially 3.0 to 4.0 mol/l andparticularly about 3 mol/l.

The anode material used can be in principle any electrode material whichis stable under the process conditions, namely for example:

Vitreous graphite, lead, lead alloys, noble metals such as platinum ormetals which are passive towards anodic corrosion (e.g. zirconium andtantalum) and which are coated with a noble metal (e.g. with palladiumor ruthenium).

Lead alloys, for example those with a content of silver (e.g. Chrominfrom Blasberg, GFR) are especially preferred. In the electrolytes thereare found in this manner Ag ions which act as the catalyst, namely thelower valued metal ions of a transition metal-redox pair with anoxidation potential greater than Mn²⁺ /Mn³⁺. It has been found that theuse of such lead alloys is especially economical, since the life of theelectrodes is long (e.g. it can amount to 1 to 11/2 years).

As the cathode material there is likewise used vitreous graphite, lead,lead alloys, noble metals such as platinum, but especially also lead orlead alloys (e.g. Chromin).

The reaction can be carried out in an undivided cell or in a cell whichis divided by a porous diaphragm consisting of usual inert materials.The reaction is preferably carried out in a cell without a diaphragm.Although in the latter case it is convenient to carry out the reactionin a protective gas atmosphere (e.g. under nitrogen) in order to protectagainst explosive gas formation (from anodically-formed oxygen andcathodically-produced hydrogen), the gases which result in the reactionduring the electrolysis can also simply be rarefied by the addition of aprotective gas (e.g. nitrogen); however, rarefaction with air also leadsto the prevention of the danger of explosion.

For the electrochemical oxidation there can be used essentially anyconventional, especially commercially available, type of cell. Thus, forexample, there can be used

channel cells [flow cells] (in which the electrodes are arranged as acomb or as a package (stack) (in the form of plates, cylinders etc),these cells being preferred;

filter press cells (consisting of frames and plates);

trough (tank) cells (the stirring action required being realized byelectrolyte circulation, by means of an inert gas or by means ofrotating electrodes, etc.).

A preferred type of channel cell is, for example, that which has beendescribed by D. Pletcher in Industrial Electrochemistry, Chapman & Hall(London, New York), (1982), 162.

As the cell material there can be used any inert material, butespecially a synthetic polymeric material (e.g. polypropylene).

The current densities in the reaction in accordance with the inventionconveniently lie at 100 to 600 mA/cm², especially at 300 to 500 mA/cm²(i.e. 3 to 5 kA/m²).

The cell potential adjusts itself as a function of the composition ofthe electrolyte, the temperature and the geometry of the particularcell. It can, for example, assume a value of 2.5 to 4 V, especially of2.5 to 3.0 V and particularly of about 3 V.

The oxidation of p-tert.butyltoluene is carried out in accordance withthe invention with an electrochemically-generated Mn³⁺ salt, but in aseparate reaction vessel.

Thus, the reaction is preferably carried out in a three-phase system,namely: sulphuric acid/Mn³⁺ (and Mn²⁺) salt suspended therein/organicmaterial.

The sulphuric acid concentration preferably amounts to 40% to 90%,especially 50% to 65% and particularly about 55%.

The content of manganese salts [Mn⁺² (traces) and Mn⁺³ ] in theinorganic phase is conveniently at least 2.5 mol/l, thus, for example,it amounts to 3 to 4 mol/l, especially about 3 mol/l.

The organic material consists mainly of TBT, TBB and solvent. Assolvents there come into consideration especially aliphatichydrocarbons--e.g. heptanes, octanes, cyclohexane--as well as theirchlorinated or fluorinated derivatives, e.g. methylene chloride,tetrachloro ethylene, perfluoro octane (C₈ F₈), etc. These solvents havea sufficient dissolving power for TBT and its oxidation product; theyare insoluble in the inorganic phases; their chemical stabilityvis-a-vis the reaction medium is satisfactory, and they can be separatedreadily from the reaction products. Octane and chlorinated aliphaticsare preferred solvents. TBT is the especially preferred solvent.

The reaction is conveniently carried out at a temperature of 80° C. to110° C., especially 90° C. to 100° C. and particularly at about 95° C.

It is advantageous to discontinue the chemical oxidation after a 40% orlower conversion, especially after a 20% to 30% conversion, ofp-tert.butyltoluene, particularly in view of the danger of a loss ofselectivity. This amounts in the optimum case to above 90%. Possiblebyproducts are, in particular, small amounts (e.g. about 3%) of thecorresponding carboxylic acid.

The separation of the reaction product consisting essentially of TBB,MnSO₄, sulphuric acid and orgaic solvent into two inorganic phases(suspension of MnSO₄.H₂ O in sulphuric acid) and one organic phase canbe carried out by filtration and decantation or by centrifugation. Theseparation of TBB and remaining organic material or solvent isconveniently carried out by distillation.

The TBT can thereupon be recycled.

Before recycling the inorganic phase, it is conveniently freed fromresidual organic material by means of steam and/or a gas. The easiestmethod is by stripping in columns in a countercurrent procedure.

The electrochemical oxidation is preferably carried out continuously.The chemical oxidation can be carried out continuously ordiscontinuously.

The electrolytic efficiency of the electrochemical process lies at about70%.

The electrochemical oxidation of Mn²⁺ to Mn³⁺ per se is known from theliterature; see, for example, J. Electrochem. Soc. 129 [4], 749-752(1982).

It is also known that p-tert.butyltoluene can be directly oxidizedelectrochemically to p-tert.butylbenzaldehyde; see, for example,Japanese Patent Application Nos. 79/096296 and 79/56996 and DOS No.2948455.

Further, it is known that aromatic hydrocarbons (e.g. xylenes ortoluenes) can be oxidized using electrolytically-produced Mn₂ (SO₄)₃ ;see, for example, German Patent Specification No. 175295, U.S. Pat. Nos.4,212,710 and 4,212,711 and J. Electrochemical Society 110 [3], 202-204,(1963).

The parameters enumerated above, which enable the aldehyde obtainable inaccordance with the invention to be manufactured selectively on atechnical scale in an economical manner (i.e. inexpensively) can not beconcluded from any of the aforementioned literature references. In thisrespect, the present process is also clearly superior to the processusing Ce⁴⁺ salts (see, for example, German Patent Specification No.3028757). In comparison with this process, the following parameters inparticular permit an economical process: lower atomic weight, price,electrolytic efficiency, concentration of the salt in theelectrochemical oxidation˜concentration in the chemical oxidation,stability of the solvent in the anode compartment, lower cell voltage,higher concentration of the salt during the electrochemical oxidation inthe case in accordance with the invention, working-up of the(chemically) manufactured tert.butylbenzaldehyde. A particular advantageof working with manganese salts is also to be seen, in that in the scopeof the present invention it has been found that Mn³⁺ is approximately 10times less soluble than Mn²⁺ in about 50% sulphuric acid; in otherwords, the danger of the reduction of Mn³⁺ at the cathode is very muchless during the electrochemical oxidation.

Thus, while the prior art provides a number of approaches to theproblem, it does not provide a method which is selective and efficientenough to be commercially attractive for the manufacture of the aldehydeon a large scale.

EXAMPLE 1

Apparatus: undivided circulation-plate cell and container having acapacity of 1.5 l (channel cell).

Anode and cathode: Chromin (Blasberg), surface 182 cm².

Electrolyte:

454 g MnSO₄.H₂ O

428 g H₂ O,

699 g H₂ SO₄ (100%).

Temperature: 95°-100° C.

Amperage: 39 A.

Voltage: 2.5-3.0 V.

After electrolysis with a current amount of 3.58 F (96 A.h), there areobtained 725 g of Mn₂ (SO₄)₃.H₂ SO₄.4H₂ O, the electrolytic efficiencyamounts to 71.5% and the conversion relative to Mn²⁺ is 95%.

The electrolyte is now added to a reaction vessel having a capacity of1.5 l. 135 ml of water are added thereto while stirring within 10minutes and the mixture is heated to 90° C. 360 g of TBT, heated to 60°C., are now added thereto in one portion, the temperature firstlydropping to 80° C. and thereupon rising to 95° C. The volume of themixture amounts to 1.4 l. The mixture is stirred at 95° C. for 15minutes and the liquid phases are sucked through a glass frit into aseparating funnel which is thermostatized at 95° C. After separating theliquid phases, the inorganic phase is led back into the reaction vessel,stirred briefly and again sucked off. After three-fold sucking off,there are obtained 345 g of organic phase and 1.71 kg of inorganicphase, the latter containing a small amount of organic impurities.

The organic phase is washed with 180 g of a 5% Na₂ CO₃ solution(containing 5% NaCl) and with 90 g of a 10% NaCl solution. The organicphase is distilled under a vacuum on a Vigreux column and gives 270 g ofTBT as the lower boiling component and 83.6 g of TBB as the higherboiling component.

Yield:

TBT consumption: 360-270=90 g.

TBB distilled: 83.6 g (corresponding to 85% of theory).

Overall electrolytic efficiency: 71.5.80.86=57.8%.

After distilling off about 125 g of water and a small amount of organicmaterial, the inorganic phase can be let back into the electrolysiscycle in place of fresh electrolyte.

EXAMPLE 2

Apparatus: undivided circulation-plate cell and container having acapacity of 1.5 l

Anode and cathode: Chromin (Blasberg), surface 182 cm².

Electrolyte:

454 g MnSO₄.H₂ O,

428 g H₂ O,

699 g H₂ SO₄ (100%)

Temperature: 95°-100° C.

Amperage: 39 A.

Voltage: 2.5-3.0 V.

After electrolysis with a current amount of 3.58 F (96 A.h), there areobtained 725 mg of Mn₂ (SO₄)₃.H₂ SO₄.4H₂ O, the electrolytic efficiencyamounts to 71.5% and the conversion relative to Mn²⁺ is 95%.

360 g of TBT are heated to 90° C. in a 1.5 l reaction vessel. It is nowadded while stirring well within 1 to 1.5 hours to the oxidizedelectrolyte. 135 ml of water are added simultaneously to the reactionvessel. The temperature is held at 95° C. during the addition and thesubsequent reaction time (40 minutes). The reaction mass is cooled to60° C. and the liquid phases are thereupon sucked through a glass fritinto a separating funnel which is thermostatized at 60° C. Afterseparating the liquid phases, the inorganic phase is led back into thereaction vessel, stirred briefly and again sucked off. After three-foldsucking off, there are obtained 345 g of organic phase and 1.71 kg ofinorganic phase, the latter containing a small amount of organicimpurities.

The organic phase is washed at 60° C. with about 180 g of a 20% neutralNa₂ SO₄ solution. This solution can be used several times by alwaysbringing its pH-value to 7±0.5 with a small amount of 12% sodiumhydroxide solution. The organic phase is distilled under a vacuum on aVigreux column and gives 270 g of TBT as the lower boiling component and86.0 g of TBB as the higher boiling component.

Yield:

TBT consumption: 360-270=90 g.

TBB distilled: 86.0 g (corresponding to 87% of theory).

Overall electrolytic efficiency: 71.5.83.2=59.5%.

After distilling off about 125 g of water and a small amount of organicmaterial, the inorganic phase can be led back into the electrolysiscycle in place of fresh electrolyte.

EXAMPLE 3

In the plant the electrochemical oxidation Mn²⁺ →Mn³⁺ (in H₂ SO₄) can beeffected continuously in a first reaction system, and the reactionmixture so obtained then transferred into a second reaction vessel forthe realisation of the chemical oxidation TBT→TBB in the 3-phase system:sulphuric acid/Mn³⁺ salt suspended therein/organic solvent, e.g. TBT.The reaction product is separated into 2 inorganic phases (suspension ofMnSO₄.H₂ O in sulphuric acid) and an organic phase by centrifugion. Theinorganic phases are then stripped of residual organic material in apacked column of residual organic material (e.g. by steam) byapplication of the countercurrent technique, and are thereaftertransferred back to the first reaction system. The organic phase iswashed with alkali (as described above) and separated into crude TBB andorganic solvent by distillation. The TBT, TBB and water recovered bystripping are added to the system, e.g. to the second reaction vessel.

We claim:
 1. A process for the manufacture of p-tert-butylbenzaldehyde,which process comprises chemically oxidizing p-tert-butyltoluene with aMn³⁺ salt, wherein:(a) said Mn³⁺ salt is generated by electrochemicaloxidation of a Mn²⁺ salt, (b) the chemical oxidation and theelectrochemical oxidation are carried out in separate reaction vessels,(c) the chemical oxidation is carried out at temperature of 80° C. to110° C., (d) the electrochemical oxidation is carried out at atemperature of 80° C. to 100° C., and (e) the electrochemical oxidationis carried out using a current density greater than or equal to 100mA/cm².
 2. A process according to claim 1 wherein the Mn²⁺ salt ismanganese sulphate.
 3. A process according to claims 1 or 2 wherein theelectrochemical oxidation is carried out(a) in 40% (5.3 molar) to 90%(16.6 molar) sulphuric acid, and (b) with the Mn²⁺ salt present at aconcentration of 1 to 5 mol/l.
 4. A process according to claim 3 whereinthere is used a cell potential of about 2.5 to 4 V.
 5. A processaccording to claim 1 wherein the electrochemical oxidation is carriedout using(a) an anode made from vitreous graphite, lead, a lead alloy, anoble metal or a metal which is passive to anodic corrosion and which iscoated with a noble metal, and (b) a cathode made from vitreousgraphite, lead, a lead alloy, or a noble metal.
 6. A process accordingto claim 5 wherein(a) the anode material is a lead alloy, and (b) thecathode material is lead or a lead alloy.
 7. A process according toclaim 6 wherein the anode and cathode consist of the same material.
 8. Aprocess according to claim 7, wherein a lead-silver alloy is used as theelectrode material.
 9. A process according to claim 8 wherein the Mn²⁺salt is manganese sulfate.
 10. A process according to claims 5, 6, 7, 8,or 9 wherein the electrochemical oxidation is carried out in anundivided electrolysis cell.
 11. A process according to claim 10 whereinthe reaction is carried out under an inert atmosphere.
 12. A processaccording to claim 8 wherein the cell is a filter press cell, a troughcell or a channel cell.
 13. A process according to claim 12 wherein thecell is a channel cell.
 14. A process according to claims 12 or 13wherein the cell consists of an inert synthetic polymeric material. 15.A process according to claim 1 wherein the oxidation ofp-tert-butyltoluene is carried out in a three-phase system wherein twoof the three phases consists of a suspension of solid Mn³⁺ salt, or Mn²⁺and Mn³⁺ salt in sulphuric acid which suspension corresponds to thesystem Mn³⁺ (Mn²⁺)/H₂ SO₄ of the electrochemical oxidation and the thirdphase is an organic material.
 16. A process according to claim 15wherein(a) the concentration of Mn³⁺ salt or Mn²⁺ and Mn³⁺ salt isgreater than or equal to 2.5 mol/l, (b) the concentration of thesulphuric acid is from 40% (5.3 molar) to 90% (16.6 molar), and (c) theorganic material is p-tert-butyltoluene.
 17. A process according toclaim 16 wherein the reaction is carried out under an inert atmosphere.18. A process according to claim 17 wherein(a) the concentration of Mn³⁺salt or Mn²⁺ and Mn³⁺ salt is 3 to 4 mol/l, (b) the concentration ofsulphuric acid is 50% (7.1 molar) to 65% (10.3 molar), (c) thetemperature is from 90° C. to 100° C., and (d) the inert atmosphere isnitrogen.
 19. A process according to claims 15, 16, 17 or 18 wherein theoxidation is discontinued at a 40% or less conversion ofp-tert-butyltoluene.
 20. A process according to claim 19 wherein theconversion of p-tert-butyltoluene is 20% to 30%.
 21. A process accordingto claim 19 wherein(a) the reaction products are separated into theinorganic phases and organic phase by means of filtration, decantationor centrifigation, (b) the inorganic phases are treated with steam or aninert gas to remove organic materials, and (c) the inorganic phases arerecycled.
 22. A process according to claim 19 wherein the oxidation iscarried out continuously or discontinuously.
 23. A process according toclaim 14 wherein the electrochemical oxidation is carried out(a) in 50%(7.1 molar) to 65% (10.3 molar) sulphuric acid, (b) with the Mn²⁺ saltpresent at a concentration of 3.0 to 4.0 mol/l, (c) with a currentdensity of 300 to 500 mA/cm², and (d) with a cell potential of 2.5 to3.0 V.
 24. A process according to claims 4, 9, or 23 wherein theoxidation is carried out continuously.